"Aftermarket weapon customization; updated firing mechanism converts the NSG 23 burst fire mode from three rounds to five rounds."
W-Y SR4 Burst Fire Modification description (from Aliens: Colonial Marines - Arsenal Upgrades - W-Y NSG 23 Assault Rifle - W-Y SR4 Burst Fire Modification)
The SR4 Burst Fire Modification is a firing mechanism manufactured by the Weyland-Yutani Corporation for the NSG 23 Assault Rifle.